This invention relates to processes for removal of sulfur oxides (SOx) in waste gases with the use of magnesium hydroxide as a principal absorbent. More specifically, the invention relates to a process of this character wherein desulfurization is carried cut with magnesium hydroxide as an absorbent, and the SOx thus absorbed is recovered as gypsum of good quality.
The process of causing an aqueous slurry for absorbing of a calcium compound such as calcium hydroxide or calcium carbonate to contact a waste gas containing SOx in an absorption tower thereby to fix the SOx as calcium sulfite and thereby to carry out desulfurization is known. If this calcium sulfite is oxidized, gypsum will be obtained.
Since this process is accompanied by a number of problems, we have previously proposed an improved process as disclosed in the specification of Japanese Patent Application No. 30279/1973, Japanese Patent Publication No. 11680/1977, wherein magnesium hydroxide is jointly used as an absorbent in a specific proportion. This magnesium hydroxide also reacts with the SOx to become magnesium sulfite, which is oxidized at the same time as the oxidation of the calcium sulfite to become magnesium sulfate. This calcium sulfate is caused to react with the magnesium hydroxide and becomes magnesium hydroxide by a double decomposition as indicated by the equation set forth below, and, at the same time, gypsum is formed. ##STR1##
In a preceding invention of ours, the mole ratio of Ca(OH).sub.2 /Mg(OH).sub.2 in the absorbing slurry is limited to 0.5 to 99. The reason for this is as follows.
Since a magnesium compound has a higher SOx absorbing efficiency than a calcium compound, it would appear that a small value of this mole ratio is desirable if only the desulfurization efficiency were to be considered. However, a small value of this mole ratio gives rise to a large quantity of the Ca(OH).sub.2 consumed in the reaction indicated by the above reaction equation of the supplemented Ca(OH).sub.2 and, consequently, results in the formation of a large quantity of CaSO.sub.4 in the Mg(OH).sub.2 regeneration reactor. However, by a reaction according to the reaction equation (1), only a coarse or crude gypsum in muddy state can be obtained. As a consequence, the quantity of the crude gypsum in muddy state formed in the Mg(OH).sub.2 regeneration reactor becomes considerably more than the quantity of gypsum formed in the subsequent oxidation step, and this results in a lowering of the quality of the recovered gypsum.
In order to solve this problem arising from the contradictory nature of improvement of absorptivity of the absorption liquor and impairment of quality of the gypsum produced, we have previously invented a waste gas desulfurization method wherein, by limiting the mole ratio of Ca(OH).sub.2 /Mg(OH).sub.2 in the absorption liquor supplied to the absorption tower to 0.5 to 99, the absorption efficiency of magnesium compounds is sustained, whereby gypsum of good quality is obtained, as disclosed in the Japanese Patent Publication hereinbefore.
As mentioned hereinbefore, however, a magnesium compound such as Mg(OH).sub.2 has a higher SOx absorption efficiency than a calcium compound such as Ca(OH).sub.2. Accordingly, it would be highly profitable if, by using a greater quantity of Mg(OH).sub.2 than Ca(OH).sub.2, preferably by using substantially only Mg(OH).sub.2, for the absorbent, a gypsum of good quality could be obtained.